Using sensorless brushless motors and their respective controllers in dosing applications is a particularly difficult application but one which can be carried out successfully with a little care and attention.
There’s a number of ways you can set up for successful brushless dosing of which flow meters are certainly one. Don’t forget however that using a flow meter can be expensive and complex. This is particularly true when one considers that there are other alternatives which are worth investigating first….
THE SIMPLEST BRUSHLESS DOSING SOLUTION – DOSING BY TIME…
The quickest and simplest way of setting up a sensorless brushless ESC or sensorless brushless motor controller for dosing applications is to use time. In essence this can be done through an automated system which will calibrate a specific liquid through a specific tube diameter over time. Once the amount dosed/time is known it is then possible to calibrate the system using a simple control mechanism so that if a certain volume is required, the controller will know that it needs to run for a certain amount of time. For example, if the calibration process has worked out that 10s will deliver 100ml of a particular substance then a user could input 75ml into the controller and it would run for 7.5 seconds.
The key issue with using time as the key ‘measurable’ in a dosing application is that time is not volume – in other words we are not accurately measuring the volume of a particular substance but are approximating volume by time. To use an example, if one considers a large barrel of a particular chemical – for example chlorine – which is being dosed into a swimming pool at regular intervals to ensure the water remains fit for human use then the output of this barrel must be at the bottom of the barrel.
As the barrel reduces the pressure on the fluid will reduce. Whereas the first few doses will have been under additional pressure (as a result of gravitational pressure), the last few doses require careful pumping to be delivered. If time were used as the key variable by which the volume was determined, it is therefore quite likely that there may be a significant difference between the first dose out of the barrel (which would have been heavily gravity assisted) and the final dose (which would potentially have had gravity opposing it).
The result is a changing volume as the pressure changes.
NOTE: it is worth highlighting here that in applications where the input pressure on a pump is stable, time can be a very good measure provided all other factors remain constant (such as tube diameter, distance pumped etc.). However, in practice most applications have changing pressure of some sort so time will never be quite as accurate as other methods
The ZDBL Series of Sensorless Brushless Motor Controllers use back-EMF most commonly as a means of determining (and potentially adjusting) the speed of the motor. However the ZDBL BLDC Motor Controllers can also be set up to count the rotations of the motor which can therefore give a much more accurate indication of volume (depending on the type of pump being used) because if we know the volume of the pump and the rate it pumps at then we can determine that a set volume requires a set number of turns. In a test carried out with an OEM diaphragm pump on dosing 50ml using this method we generated a margin of error of 1-1.5%.
A key point to make in relation to this method is that it relies on a solid back-EMF signal in order to be able to accurately measure the range of dosing so it will not necessarily be suitable for applications where lower speeds are essential. Where you require lower speeds we would strongly recommend stepper motors and controllers as a method as they are much better suited to lower speeds. However, for pumps where stepper motors are not practical then this method is worth thinking about
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